1. Field of the Invention
The present invention relates to a nuclear reactor building that houses a pressure containment vessel (PCV) of a nuclear power plant and a construction method thereof.
2. Description of the Related Art
A conventional nuclear reactor building (or merely reactor building, called hereinafter) of a nuclear power plant has been constructed by a frame made of reinforced concrete in order to meet earthquake resistant design criteria and achieve radiation shielding. The reinforced concrete building is constructed by first arranging a grid of reinforcing bars in a formwork and then pouring concrete into the formwork. After completion of the construction of the building, a pressure containment vessel and other equipment are installed in the building. These works are carried out mainly at the site of the construction of the plant, so that on-site construction work takes long time, and it may sometimes take four or more years to construct the whole of a nuclear power plant.
Recently, it is strongly demanded to reduce time and cost for constructing a building of a nuclear power plant, and in order to reduce the construction period, it is contemplated to eliminate the reinforcing bar arranging work and the concrete casting work by adopting a steel plate reinforced concrete structure composed of a combination of a steel plate and concrete (referred to as an SC structure hereinafter) or a steel structure composed mainly of steel members to construct the building, rather than the conventional reinforced concrete structure.
However, reducing only the period of construction of the frame of the reactor building is not enough to reduce the total period of construction of the entire nuclear power plant, and it is also required to reduce the period of installation of various equipments into the building. In view of such circumstances, recently, there has been provided a construction method that reduces the construction period by applying the SC structure to the frame of the reactor building, in which a composite module composed of an SC steel plate structure serving as the formwork of the frame and piping and other equipment installed or mounted on the structure is previously manufactured, and then the composite module is placed at the site of installation (see Patent Document 1: Japanese Patent Laid-Open No. 2003-167086, for example).
In addition, requirements on the security of the nuclear power plant have become severe in recent years, and there is a demand for including provision not only for design basis accidents but also for unexpected events, such as severe accidents, in the design criteria of plant facilities of nuclear power plants. For example, if a loss of coolant accident (LOCA), which is a design basis accident, occurs, and reactor cooling fails, hydrogen may be generated in the PCV, increasing the pressure in the PCV beyond the design value.
In such a case, for a pressure suppression type PCV, a steel PCV is more advantageous than a reinforced concrete PCV in points that thermal deterioration of concrete does not occur and the surface of the PCV can be externally cooled, for example. Thus, it is expected that the superiority of the steel PCV will be appreciated in the future.
The reactor building needs a biological shielding wall (which may be called BSW hereinafter) serving as a radiological countermeasure that is made of reinforced concrete and placed outside the steel PCV at a certain distance (1 m or less) so that the wall is not in contact with the PCV. In construction of the reactor building having the steel PCV, the PCV and the surrounding BSW need to be separately independently constructed. Besides, the PCV has outwardly protruding PCV penetration portions, such as pipes for external connection, and in addition, it is also needed to form openings, in the BSW, for the PCV penetration portions so as to pass through the BSW in such a manner that the openings are not in contact with the PCV penetration portions, but the gap between the openings and the PCV penetration portions is made minimal so as to ensure the radiation shielding effect.
Thus, when the steel PCV is used, in order to ensure the positional relationship between the PCV and the BSW, the PCV has to be constructed before the construction of the concrete frame of the BSW, and therefore, a downtime waiting for completion of the construction of the PCV may occur.
According to another recent method for reducing the construction period based on building modularization, the BSW is constructed as an SC building module by applying the SC structure to the BSW and integrating the BSW with the SC building. In this case, the SC building module is installed vertically from the upper side, which may cause a case that the SC building module inevitably interferes with the PCV penetration ports extending from the PCV.
In this regard, the reinforced concrete PCV and the steel plate concrete PCV are advantageous over the steel PCV. These PCVs combine the pressure resistant and confinement capabilities of the steel PCV and the shielding capability of the BSW and do not have the above-described defects or problems with the PCV penetration portion of the steel PCV because the PCV penetration portion and the BSW are previously integrated.
A structure of a reactor building of a nuclear power plant and a construction method thereof according to the prior art will be described hereunder with reference to the accompanying drawings.
FIG. 7 is a cross-sectional view of a reactor building under construction according to the prior art. This drawing illustrates modularization at the time of the reactor building being constructed.
FIG. 8 is a diagram for illustrating mounting of SC building modules of the reactor building.
In FIG. 7 a reactor building 2 is constructed of steel PCV blocks 60 and steel plate concrete (SC) building modules 80 and houses a steel pressure containment vessel (PCV) 1. PCV penetration portions 3 protrude to the outside of the PCV and penetrate a biological shielding wall (BSW) 4 made of reinforced concrete disposed outside the PCV. The steel PCV 1 is divided into steel PCV blocks 60 (A to F) by horizontal dividing planes 50, and the reactor building 2 is divided into SC building modules 80 ((i) to (iv)) by horizontal dividing planes 70. FIG. 8 shows interference of an SC building module 80 with the PCV penetration portion 3 of a steel PCV block 60.
As described above, in the construction of the reactor building and the steel PCV according to the conventional SC building modularization, the modules are mounted vertically from the upper side. Therefore, the SC building module inevitably interferes with the PCV penetration port protruding form the PCV. This poses an obstacle to reduction in the construction period, and a solution thereto has been required.